Since several years, vinylidene fluoride based fluoroelastomers have successfully found application in the energy, oil and gas industry, thanks to their outstanding mechanical properties in a broad temperature operating window and to their chemical resistance. They have been thus used as sealants or flexible components in diverse equipments like notably valves (ball valves, check valves, control valves), separation equipments, connector systems, measuring, gauging and monitoring equipments, downhole service tools and the like.
With oil supply scarce, drilling operations must enter more and more hostile environments; with more and more chemical being injected, higher pressures and hotter bottom hole temperatures, still combined with a continuous need for longer life expectancy for all components, a continuous need exist for seals having improved performances.
One of the challenges to be addressed is the problem of explosive decompression (E.D.): this term is used to encompass conditions occurring after an elastomer part is exposed to a fluid under extremely high pressures; these conditions are e.g. encountered when a high pressure valve is suddenly opened or when a flexible gas hose is disconnected. Structural failures in the form of blistering, internal cracking and splits might result when the fluid pressure is suddenly reduced. Actually, the elastomeric components of a system are, to a less or greater extent, susceptible to the permeation and diffusion of fluids, and more particularly gases, dissolving through their surfaces. With time, these components will become saturated with said fluids: as long as the internal gas pressure of the elastomer remains in equilibrium with the surrounding pressure, there is minimal damage (if any) and no deterioration of the performances of the elastomeric component occurs—unless caused by other factors, such as chemical or thermal degradation or by extrusion damage.
However, when the external gas pressure is removed, or pressure fluctuations occur, large pressure gradients are created between the interior and the surface of the elastomeric component, and gas diffused in said component expands and moves toward the surface. If said gradient exceeds the biaxial expansion capabilities of the component, fractures or ruptures will occur through formation of cracks and growth of blister.
Due to their behaviours in above mentioned conditions, and to the relatively poor performances of traditional fluoroelastomer grades, hydrogenated nitrile-butadiene rubbers have been identified as the materials of choice for fields of uses wherein E.D. concerns are key issues; nevertheless, these materials suffer from narrower operating window, in particular for poor performances at temperatures exceeding 200° C. and from poor chemical resistance with respect to certain chemicals.
For meeting the challenge of E.D., but also for ensuring higher continuous service temperatures (generally exceeding 200° C.), and for withstanding a broader range of chemicals, fluoroelastomers possessing improved chemical resistance and mechanical properties, in particular at high temperatures, while still possessing outstanding sealing capabilities, are required, which can furthermore ensure the requested resistance to explosive decompression.
On the other side, fluoroelastomers possessing relatively high molecular weight have been previously described. More particularly, EP 0967248 A (AUSIMONT SPA) 23 Jun. 1998 discloses in its example 3B a VDF/HFP/TFE terpolymer (53.5% moles; HFP 23.5% moles; TFE 23% moles) comprising recurring units derived from bis-olefin and iodinated end-chains derived from the use of 1,6-diiodoperfluorohexane as chain transfer agent (iodine content=0.19% wt), this terpolymer possessing a Mooney viscosity ML (1+10) at 121° C. to ASTM D1646 of 80 Mooney units. This component is used as high Mooney fraction in a multimodal molecular weight composition further comprising a low Mooney and a middle Mooney fraction, so as to yield a compound having a Mooney of 37. In words, high Mooney fluoroelastomers were generally used for formulating fluoroelastomer compositions in admixture with lower molecular weight materials, so as to ensure adequate processability. Nevertheless, these compositions loose their mechanical performances already for temperatures of 100° C. or more, so that they cannot deserve the oil and gas use requirements.